ROLE OF THE VAPOR PHASE IN THE WELDING, CRYSTALLIZATION, AND COOLING OF IGNIMBRITES AT YUCCA MOUNTAIN, NEVADA, AND KINGMAN, ARIZONA

The Tiva Canyon and Topopah Spring Tuffs at Yucca Mountain, Nevada, and the Peach Springs Tuff at Kingman, Arizona, contain similar lithostratigraphic features indicating a petrogenetic sequence of these features influenced by the vapor phase. These mid-Miocene ignimbrites vary from nonwelded to densely welded, are vitric near the top and bottom but crystallized in most of the deposit, and have lithophysal and nonlithophysal zones. Porosity varies from 30 to 50 percent in nonwelded tuff and from 2 to 5 percent in densely welded vitric tuff. Vapor trapped during deposition was compressed and redistributed during welding through the matrix and along fractures. Some vapor escaped the deposit, but much was trapped to form lithophysae that have various shapes, indicating growth in a ductile material and vapor at super-lithostatic pressure. A typical lithophysa consists of a cavity lined with vapor-phase minerals (mostly tridymite), a light-gray rim (commonly spherulitic feldspar with cristobalite or tridymite), and a reddish-purple border (feldspar and quartz or cristobalite). Lithophysae are typically surrounded by a weak-red matrix-groundmass that is slightly more coarse-grained than the surrounding brown matrix-groundmass. These features indicate the vapor in a lithophysal cavity reacted with the surrounding glass to crystallize a rim, and then reacted to gradationally lesser amounts to form a border, a weak-red matrix-groundmass, and possibly no vapor to form a brown matrix-groundmass. Expansion cracks in some lithophysae indicate continued growth of cavities while rims crystallized. Some cavities merge into veinlets in the host rock with rims, borders, and weak-red matrix-groundmass. Fractures with similar rim, border, and matrix-groundmass crystallization indicate formation in glass, whereas those without formed after the glass crystallized. The vapor changed with time; early vapor could corrode glass, some vapor was produced when glass crystallized, and vapor-phase minerals formed on walls of many lithophysal cavities and fractures during cooling of the deposit. These welding, crystallization, and cooling processes were influenced by the vapor phase and result in a petrogenetic sequence that formed prior to complete cooling of the deposits and likely is applicable to other ignimbrites.